1. Field of the Invention
This invention relates to a resistive liquid level/temperature sensor and transmitter including low voltage impedance measurement circuitry and a liquid level controller to convert three impedance values into indications of the temperature and the levels of two different liquids (e.g., oil and water) having two specific gravities located within an (e.g., oil) storage tank. The foregoing is accomplished, in part, by means of a resistive chain network that extends continuously within a probe that is dipped into the liquids contained by the tank.
2. Background Art
Resistive level sensors are well known for measuring the level of a single liquid such as oil, chemical fluid, or waste water in a containment vessel, such as a tank. One such level sensor 1 is shown at FIG. 1 of the drawings. The level sensor 1 includes a resistive chain network containing an array of series connected resistors RES1 . . . RES5 and a corresponding array of parallel connected, normally-open reed switches RS1 . . . RS5. The number of resistors and switches shown in FIG. 1 is for the purpose of illustration, and the actual numbers will depend on the length of the level sensor. The resistive network is loaded into an elongated hollow cylindrical probe 3. A magnetic float 5 having a magnet inside surrounds the cylindrical probe 3. probe 3. The probe 3 is dipped into the liquid to be measured, and the float 5 rides up and down the outside of the probe depending upon the level of the liquid in the containment vessel. The particular location of the magnetic float 5 along the cylindrical probe 3 causes an adjacent one of the normally-open reed switches (e.g., RS2) to close and thereby complete a circuit between the network excitement voltage (Vs) and the network output (VOUT) by way of the network resistors (e.g., RES1+RES2).
The resistive chain network of the resistive chain level sensor 1 of FIG. 1 functions as a voltage divider to indicate the level of the liquid into which the probe 3 is dipped. The excitement voltage VS is provided to the network and the voltage output VOUT is measured. However, the level sensor 1 of FIG. 1 is only capable of indicating a single liquid level by calculating the ratio of the output and excitation voltages.
Moreover, the output impedance of the resistive chain network of the level sensor 1 is very high and irregular. The maximum output impedance is one half the total impedance of all of the resistors RES1 . . . RES5 and switches RS1 . . . RS5. Such a high output impedance decreases accuracy, and the variant impedance introduces non-linear errors to the level calculation. The typical analog input to the level sensor 1 requires an extremely high impedance to be able to read the output voltage of the resistive chain network. What is more, the analog input cannot support a precision temperature sensor (e.g., a resistive temperature device) in cases where knowing the temperature of the liquid to be measured is important.
For some industrial applications, having access to accurate dual liquid level and temperature measurements is highly desirable. That is to say, it would be advantageous to be able to measure the levels of two different liquids lying one above the other in a single storage tank as well as the temperature at the bottom of the tank. It would also be advantageous to be able to accomplish the foregoing by means of a resistive liquid level sensor and transmitter which takes relatively low impedance measurements and consumes relatively low power when compared with the conventional resistive chain level sensor described above.